I. Field of the Invention
The present invention relates to medical devices. More particularly, the present invention relates to a sheath for use with medical devices.
II. Description of the Related Art
The use of minimally invasive surgical techniques has dramatically affected the methods and outcomes of surgical procedures. Physically cutting through tissue and organs to visually expose surgical sites in conventional "open surgical" procedures causes tremendous blunt trauma and blood loss. Exposure of internal tissues and organs in this manner also dramatically increases the risk of infection. Trauma, blood loss, and infection all combine to extend recovery times, increase the rate of complications, and require a more intensive care and monitoring regiment. The result of such open surgical procedures is more pain and suffering, higher procedural costs, and greater risk of adverse outcome.
In contrast, minimally invasive surgical procedures cause little blunt trauma or blood loss and minimize the risk of infection by maintaining the body's natural barriers to infection. Minimally invasive surgical procedures result in faster recoveries and cause fewer complications than conventional surgical procedures. Minimally invasive procedures, such as laparoscopic, brachytherapy, endoscopic, or cystoscopic surgery, have replaced more invasive surgical procedures in many areas of medicine. Due to technological advancements in areas such as fiber optics, micro-tool fabrication, noninvasive visualization, and material science, more easily operated and more cost effective tools are available to the physicians for use in minimally invasive procedures. However, a host of technical hurdles still exists that limit the efficacy and increase the difficulty of minimally invasive procedures.
One critical aspect of minimally invasive surgical techniques is the ability of the operator to visualize the position of surgical instruments within the body and to determine the extent of the manipulation of organs and tissues caused by the surgical instruments. For example, percutaneous coronary angioplasty (PTCA) uses fluoroscopy to position a tiny balloon at the end of a long flexible catheter in a coronary artery where a stricture has reduced blood flow to the heart. Another example of a minimally invasive procedure is arthroscopic surgery in which fiber optic visualization is used to position and control small tools within bone joints to repair, ablate, or remove tissue.
Fiber optic and fluoroscopic visualization require a vessel, duct, or cavity into which a transparent or radiopaque fluid can be injected. However, procedures not involving a vessel, duct, or cavity where a fluid can be injected and contained require other methods of visualization. For example, the manipulation of soft tissue organs requires entirely different methods of visualization. In such procedures, visualization techniques such as magnetic resonance imaging (MRI) or ultrasound which distinguish the borders and shapes of soft tissue organs or masses are used.
MRI has been particularly effective in providing detailed visualization of damage or growth of soft tissues surrounded by other organs and structures. MRI measures the radio frequency (RF) signals emitted by the nuclei of atoms subjected to a transient magnetic field while in a strong static field. However, the size, cost, complexity, and nature of MRI systems may make them poor candidates for visualization in many surgical procedures.
Ultrasound imaging relies on the reflection of high frequency sound waves at interfaces of varying acoustic impedance to create a two dimensional picture of internal body structures. Many prior art ultrasound imaging systems exhibit undesirable features such as a lack of structural resolution, phantom and ghost images created by scattered sound waves and limited visualization due to obstructions in the field of view. However, ultrasound does provide a lower cost, less complicated and more compact alternative to MRI. Ultrasound imaging systems can typically be operated by one person, are a fraction of the cost and size of an MRI system, are mobile, and are less restrictive on the operating environment.
Modern ultrasound technology has expanded the application of minimally invasive surgical techniques into areas in which direct surgical intervention had previously been the only option. One surgical technique to which modem ultrasound imaging has been applied is cryosurgery. Cryosurgery involves the freezing of diseased tissue. Cryosurgery has been used for decades with limited success to destroy diseased tissue throughout the body. Historically, cryosurgery has been limited in its application to the destruction of tissue on the surface of the body or in a space where the visible manipulation of tissue was possible. Recently the role of cryosurgery has been expanded to include the application of cryosurgery in a minimally invasive manner. Minimally invasive techniques were made possible by advances in the ability to visualize soft tissues by ultrasound imaging. Ultrasound imaging allows the surgeon to visualize tissue or organ "landmarks" within the patient's body and, thereby, correctly position the freezing probe or cryoprobe within the soft tissue.
The surgical procedure for destroying a target tissue with cryosurgery begins by placing the probe in the target tissue mass. Ultrasound imaging is typically used to guide the probe in a minimally invasive manner. An ultrasound transducer on the probe in close proximity to the freezing element is used to facilitate the positioning of the probe. The ultrasound transducer on the probe generates an ultrasound signal in response to detection of an incident ultrasound signal from an imaging system. The response signal is detected by the imaging system causing an image to be displayed on the monitor of the imaging system. The response signal radiates from the ultrasound transducer on the probe, thus, defining the location of the probe tip and freezing element.
Prior art ultrasound transducers use ferroelectric piezoceramic transducer elements. Ferroelectric transducers are made of materials such as lead zirconate titanate (PZT) and, therefore, are glasslike and brittle. Due to their brittle nature, ferroelectric; elements are susceptible to breaking or cracking. To avoid breakage during the surgical process, the ferroelectric transducer must be mounted securely to the medical device within a protective enclosure as an integral part of the medical device. These requirements add cost and complexity to the design of the medical device.
The resonant frequency of ferroelectric transducers over which they are useful as ultrasound transducers is determined by the thickness of the device. A typical ferroelectric element has a hollow cylindrical shape. Electrodes are placed on the inner and outer cylindrical surfaces of the transducer. Ferroelectric transducers are narrow band devices, meaning the frequency bandwidth over which a ferroelectric element is useful as a transducer is limited. For example, a ferroelectric element with a cylindrical wall thickness of approximately 0.015 inches is useful as an ultrasound transducer over a range from about 5 Megahertz (MHz) to about 10 MHz Practical manufacturability concerns constrain the minimum size of ferroelectric elements in cylindrical shapes to a cylindrical wall thickness of approximately 0.012 inches meaning that the actual diameter of the devices is significantly larger. Inclusion of an element with such a relatively large size can adversely impact the design of the medical device and interfere with the efficient operation of the device.
A variety of different medical devices may benefit from the inclusion of an ultrasound transducer. The task of redesigning the large variety of medical instruments to include an ultrasound transducer is quite burdensome, requiring extensive redesign, retesting and manufacturing retooling. Therefore, the inclusion of an ultrasound transducer on a large variety of medical device may be financially prohibitive.
In addition, the presence of an ultrasound transducer is not required for all procedures which might be carried out by a particular medical device. Therefore, inclusion of an unnecessary ultrasound transducer can add cost and complication to a medical device as well as sub-optimal design constraints. Therefore, there are many situations in which it is advantageous to use medical devices which do not contain an ultrasound transducer.
Therefore, it is apparent that there has been a long felt need in the industry to have an efficient means and method of providing ultrasound capabilities to a variety of medical instruments as needed without significantly interfering with the performance of the device. The present invention provides an elegant means and method for satisfying this need.